The invention relates generally to ventilators for supporting ventilation in air breathing mammals, both humans and animals. More particularly, the present invention relates to high frequency ventilators which operate by supplying oscillatory respiratory gases to a patient at a frequency above the normal breathing frequency of the patient.
High frequency oscillating ventilators are well known and have been previously described in, e.g., U.S. Pat. No. 4,719,910, which is incorporated herein by reference in its entirety. Unlike conventional ventilators which ventilate by positive-pressured gas flow and rely on passive recoil of the lung tissue for expiration, high frequency oscillating ventilators employ an active expiratory phase in which gas is pushed into and pulled out of a patient's lungs during alternate cycles of the oscillating diaphragm (or piston) of the ventilator. The forward motion of the diaphragm (toward the patient) creates a positive-going pressure relative to the static pressure in the patient's airway. As the diaphragm is driven rearward from its most forward position, the dynamic pressure it generates reverses from positive-going to negative-going. This bipolar dynamic pressure waveform is the principal reason for the success of the high frequency oscillatory ventilator in providing improved respiratory gas exchange.
A problem that has been encountered in the use of high frequency oscillating ventilators of the type disclosed in the aforesaid U.S. Pat. No. 4,719,910 is that the volume of air it can deliver to a patient is limited, making such ventilators suitable only for young children. Consequently, older children and adults in a critical care environment may not benefit from the advantages of this high frequency oscillating ventilator.
Another problem that has been encountered in the use of high frequency oscillating ventilators is noise. Users have complained that the high frequency oscillatory ventilator is loud when compared with other equipment in the intensive care unit. This noise comes from several sources. First, air used to cool the coil exits through holes at the rear of the driver. These holes allow a sputtering sound at the driver frequency to pass unhindered to the surrounding environment from the driver. Further, the outrush of air through these holes produces additional noise. Second, ambient air rushing to the air entrainment port of the ventilator creates a constant high frequency noise. Third, the evacuation fan within the ventilator enclosure creates a whining noise. Taken together, these three factors create a significant amount of unwanted noise. This noise can be disruptive to communications between caregivers, unsettling to young patients dependent upon the device, and annoying to other patients nearby.
Finally, cotton is currently used as the material for the coil suspension elements. Over time, cotton is susceptible to mechanical fatigue, resulting in tearing and failure.